1. Field of the Invention
This invention relates to polymeric coating compositions which can be applied to a variety of substrates, and in particular, to a coating composition that is useful in automotive finishes.
2. Description of the Prior Art
A great need exists for a coating composition that can be used to repair damaged lacquer coatings, in particular, acrylic lacquer coatings which are widely used in the auto industry. A coating composition of this type would find wide use in the repair of damaged autos and trucks and in the production of autos and trucks where portions of the vehicle often require touch-up painting after assembly. Methacrylate coating compositions are well known in the art as shown by Crissey and Lowell U.S. Pat. Nos. 2,934,509 and 2,934,510, both issued on Apr. 26, 1960. Coating compositions formed from cellulose acetate butyrate and a methacrylate polymer have been successfully used on metal substrates as shown in Evans U.S. Pat. No. 2,849,409, issued Aug. 26, 1958. Polymers containing oxazoline drying oils have been used as coatings and are disclosed in Miranda et al U.S. Pat. No. 2,208,981, issued Sept. 28, 1965, and Purcell U.S. Pat. No. 3,248,397, issued Apr. 26, 1966. The coating compositions of these prior art patents are excellent for many uses but do not have the properties preferred for a repair composition.
Generally, films of the so-called methyl methacrylate lacquers prepared from polymers containing methyl methacrylate as a main monomer have excellent characteristics in such properties as colorlessness, transparency, gloss retention over a long period and resistance to yellowing.
However, when formed into a film, a methyl methacrylate lacquer obtained by polymerizing methyl methacrylate alone or in combination with a small amount of an acrylate of C.sub.2 -C.sub.16 alkanol, e.g. ethyl acrylate, n-butyl acrylate or 2-ethylhexyl acrylate, or a methacrylate of C.sub.4 -C.sub.16 alkanol, e.g. 2-ethylhexyl methacrylate or lauryl methacrylate (these monomers are so-called internal-plasticizing monomers), tends to form cracks in the film by swelling or shrinking action due to temperature variation or to moisture, and encounters difficulties in impact resistance and in adhesion onto coated substrate surfaces. Further, this kind of lacquer is liable to cause shrinking or cracking phenomenon when recoated onto its own film or overcoated onto other films. That is, the lacquer is inferior in recoating and overcoating.
In case the amounts of said plasticizable monomers are increased, the above drawbacks can be overcome to a considerable extent, but the resulting film becomes soft, and it becomes high in temperature susceptibility and thermoplastic tendency and also its gasoline resistance and water resistance deteriorates.
Even in the case of a methyl methacrylate lacquer in which an internal-plasticizing monomer has been used in such a suitable amount as to satisfy, from the standpoint of its composition, the hardness, gasoline resistance and crack resistance of the resulting film and such proportion as recoating and the like, these properties are not satisfactory, in practice, unless the molecular weight of the polymer employed is made higher than a definite limit. However, the molecular weight of a copolymer obtained by solution polymerization is closely connected with the viscosity of the copolymer solution, and therefore such methyl methacrylate lacquer cannot be applied, in general, unless it is diluted, before application, with a large amount of solvent. This indicates the fact that such a methyl methacrylate lacquer as mentioned above is not well retained on a substrate to be coated and cannot be applied thickly.
Polyurethanes constitute a broad class of polymeric materials having a wide range of physical characteristics. The polymers are usually produced through the interaction of a polyfunctional isocyanate with a polyfunctional chemical compound having an active hydrogen in its structure.
In U.S. Pat. Nos. 3,865,898, 3,257,476 and 3,291,859 there are disclosed block copolymers and processes for forming them. In U.S. Pat. Nos. 3,257,476 and 3,291,859 the block copolymers are of an A-B-A structure. These patents teach a synthesis route to the preparation of block copolymers wherein one block is a vinyl copolymer and the other block can generally be referred to as a polyurethane. In the general scheme of synthesis as taught by these patents, a prepolymer is formed by reacting an aromatic diisocyanate with a polymeric material having functional groups with active hydrogen to form an "isocyanate-capped" prepolymer. The prepolymer is then usually reacted with tert-butyl hydroperoxide, a cumene hydroperoxide, or a dual-functional free radical initiator to form a peroxycarbamate which has reactive sites capable of initiating the polymerization of ethylenically-unsaturated monomers to form a block copolymer.